Pneumatic cells have been used in the past to measure soil or water pressure. Generally, conventional pneumatic cells include an input tube ending in an orifice, and an output tube ending in an orifice, both orifices being covered by a resilient membrane that acts as a seal. In one application, the pneumatic cell is mounted on a blade for insertion into the soil, and includes a perforated outer ring and a perforated central disk, with the ring and central disk being separated by a threshold. The resilient membrane extends over the outer face of the cell in sealing engagement with the threshold to prevent communication between the perforations of the outer ring and the perforations of the central disk. The use of a conventional pneumatic cell involves the insertion of the blade into the soil such that the membrane engages the soil, or alternately, may involve the contact of fluid pressure to the outer surface of the membrane. A pressure is then applied through the apertures of the outer ring to the back side of the membrane, and is monitored by a main pressure gauge. The pressure is increased until the pressure equals soil pressure wherein the sealing engagement of the membrane with the threshold is broken, thereby allowing the pressure to release across the threshold and flow in a reverse direction through the apertures of the central disk. The detection of such leakage flow by a flowmeter indicates that the soil pressure has been equalized, adjacent the surface of the outer ring, with the reading on the main pressure gauge indicating such soil pressure.
Such conventional pneumatic cells are subject to erroneously high pressure readings. Such error exists because the pressure at the return orifice, or in the case cited above within the apertures of the central disk, remains at atmospheric pressure, thereby leaving the external soil or water pressure unbalanced in the area of the central disk and creating a tendency for the membrane to stick to the threshold.
Therefore, a primary objective of the present invention is the provision of an improved pneumatic pressure cell operating system and a method of using such system which provides accurate soil or water pressure measurements.
A further objective of the present invention is the provision of a back-pressured pneumatic pressure cell and a method of using the same wherein a first pressure is applied to the membrane through the apertures of the outer ring and a second pressure, slightly less than or greater than the first pressure, is applied to the membrane through the apertures of the central disk.
A further objective of the present invention is the provision of a back-pressured pneumatic pressure cell and a method of using the cell wherein the pressure differential between the apertures of the outer ring and the apertures of the central disk is controlled.